This invention relates generally to a technique for converting small round logs into lumber products, and more particularly to a technique in which logs of small diameter are cut into pieces that are fitted together to create superposed layers forming a block assembly, the pieces in the block assembly being interlaminated to form an integrated stock block that is dividable into usable panels.
A technique in accordance with the invention, though applicable to various species of wood, is of particular value in connection with balsa wood derived from a tropical American tree (Ochroma pyramidale). Balsa wood has outstanding properties unique in the lumber field; for on the average, it weighs less than 9 pounds per cubic foot, this being 40% less than the lightest North American species. Its cell structure affords a combination of high rigidity and compressive and tensile strength superior to any composite or synthetic material of equal or higher density. While a technique in accordance with the invention will be described herein only in regard to balsa wood, it is to be understood that it is also applicable to other wood species.
The market potential for balsa wood board is considerable; for structural sandwich laminates can be created by bonding thin facings or skins to balsa wood panels which function as a core. Thus the Kohn et al. U.S. Pat. No. 3,325,037 and the Lippay U.S. Pat. No. 3,298,892 disclose structural sandwich laminates whose core is formed of end grain balsa wood, the laminates having an exceptionally high strength-to-weight ratio as well as excellent thermal insulation properties.
End-grain balsa-cored sandwich laminates are widely used in transportation and handling equipment, such as for floors of railroad cars, shipping containers, cargo pallets, bulkheads, doors, reefer bodies, as well as in a wide variety of other applications. These laminates are also employed for structural insulation in aircraft applications, housing and in boating.
Quite apart from the structural merits of balsa, this wood is of particular value in cryogenic applications, for it has a low coefficient of expansion and hence deforms only slightly under severe temperature changes. Moreover, the k-factor of balsa wood is such as to render this material highly suitable as thermal insulation. The symbol for thermal conductivity is the k-factor, this being the amount of heat expressed in BTU's transmitted in one hour through one square foot of homogeneous material, for one inch thick, for each degree of Fahrenheit of temperature difference between opposed surfaces of the material.
As noted in my above-identified copending application whose entire disclosure is incorporated herein by reference, the cost of balsa wood products is keyed to the low yield obtainable when employing conventional techniques to convert balsa logs into usable products. The traditional conversion technique results in a low yield in that the amount of balsa convertible into usable lumber is usually less than half the total volume of wood in the log, the balance being wasted.
In the traditional process, a series of longitudinal cuts are made through the log to produce so-called "flat sawn" pieces whose broad faces lie in a plane parallel to a tangent to the curvature of the cylindrical log. Flat sawn pieces not only give rise to a substantial amount of wood waste, but such pieces tend to warp during the kiln drying process. And even when adequately dried, flat sawn pieces undergo dimensional changes as a result of variations in air moisture or relative humidity, this resulting in deformation of the final product.
In order to improve the yield obtained from cylindrical logs, it is known to cut logs into interfitting sectors and to join these sectors together to form lumber products. Among U.S. patents which disclose a process for making lumber products in this manner are the Sorensen patent 781,376, the Anderson U.S. Pat. No. 2,878,844 and the U.S. Pat. Nos. to Hasenwinkle, 3,903,943; 3,961,644 and 3,989,078.
As pointed out in my copending application, none of these prior patents discloses a high yield technique which, when applied to balsa wood, results in balsa wood panels that can be either of the end grain or flat grain type, and which makes it possible to exploit balsa logs in a broad range of diameters running between very young trees having a four-inch diameter and fully mature trees of twenty-inch diameter or greater.
Balsa trees are fast growing and reach cutting maturity within six to eight years, at which time the diameter at breast high (DBH) can be 10 to 12 inches. The technique disclosed in my copending application makes use of young balsa trees of a diameter as small as four inches that are lighter and more readily available than older and larger trees, the technique lending itself to large scale balsa production on ordinary plantations with a very rapid turnover of trees in the order of four to six years.
The present invention, as distinguished from that disclosed in my copending application, makes it possible to exploit logs taken from the branches and upper parts of trees having diameters in the range of about 11/2 to 4 inches, such branches and tree parts normally being unsuitable for the production of lumber products. In addition to such branches, the present invention makes use of very young trees whose trunk diameters lie in a range of 11/2 to 4 inches.
The economics of converting balsa logs into commercially-available lumber products must take into account a number of factors, such as growth time, kiln drying costs and the relationship of yield to tree diameter. The traditional conversion technique for producing balsa lumber products from logs having a diameter of 12 inches or greater inevitably results in products which are expensive; for it not only requires about eight years before the trees can be harvested to produce logs of this size, but kiln costs are high and the yield is low in that a large percentage of the wood is wasted in the conversion process.
A marked improvement in the economics of converting balsa logs into usable product is gained by the technique disclosed in my copending application; for in this technique, logs as small as four inches in diameter are radially cut into sectors having the same apex angle, each sector then being longitudinally sliced at its apex and arc to form a truncated piece having a trapezoidal cross-section, only a relatively small percentage of the wood being wasted. The pieces are thereafter fitted together in a complementary manner and interlaminated to form an integrated stock block which is dividable into panels.
The technique disclosed in my copending application makes it possible to commercially exploit a broad range of balsa log diameters, running from small diameter logs cut from trees which take only 9 to 10 months to grow to large diameter logs cut from more mature trees that take at least 5 to 8 years to grow. In this way, better use can be made of the available acreage. And because the logs are cut radially, the resultant area of the exposed surfaces is greater than that obtained with conventionally cut logs, thereby markedly reducing kiln drying time and its attendant costs. But even more important is the fact that the yield is exceptionally high; for, as compared to a traditional conversion which requires 60 logs of 12-inch diameter and 16 feet length to produce 1,000 board feet of balsa product, the technique disclosed in my copending application yields the same amount of product from merely 20 such logs.